Stratification is a well-known sampling tool built on the premise that like units in a population should be treated similarly. It is a statistical fact that grouping similar units, when sampling, can generally reduce [Page 595]the variance of the survey estimates obtained. Stratification can be done when selecting units for study, or it can be carried out afterward. The latter application is usually termed post-stratification.
To illustrate the differences between stratification and post-stratification, assume a researcher is interested in the total poundage of a population that consisted of 10,000 baby mice and one adult elephant. Suppose, further, that the average baby mouse weighted 0.2 pounds but the elephant weighted three and one-half tons or 7,000 pounds. This would mean, if the whole population were to be ...
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